Glycolic acid esters, lactic acid esters, and like α-hydroxycarboxylic acid esters have been used as raw materials (condensation polymerization monomers) for various synthetic resins such as polyglycolic acid and the like. Moreover, they are industrially useful compositions as cleaning agents, etchants, etc.
Processes known for producing glycolic acid esters, a type of α-hydroxycarboxylic acid ester, are, for example, those described in the following Items 1 to 4:    1. Synthesizing a polyglycolide from formaldehyde and carbon monoxide in the presence of a heteropolyacid, and then subjecting it to alcoholysis (for example, Japanese Unexamined Patent Publication No. 1994-228045, claims);    2. Producing glycolic acid from formaldehyde and carbon monoxide, and then esterifying it with an alcohol (e.g., Korean Patent Publication No. 9511114);    3. Conducting oxidative esterification between glyoxal and an alcohol in the presence of a catalyst (e.g., Japanese Unexamined Patent Publication No. 1996-104665, claims); and    4. Hydrogenating an oxalic acid diester (e.g., Japanese Unexamined Patent Publication No. 1994-135895, claims).
In view of the processes described above, the present inventors previously developed a production process employing oxidative esterification (see, for example, Japanese Patent Application No. 2002-204748, claims). In this process, an α-hydroxycarboxylic acid ester is produced by reacting a 1,2-diol with an alcohol in the presence of oxygen. This process enables an α-hydroxycarboxylic acid ester to be produced more efficiently.
However, α-hydroxycarboxylic acid esters (especially glycolic acid ester) are unstable to heat and relatively reactive. Therefore, even in the production process disclosed in the aforementioned Patent Application No. 2002-204748 (claims), the desired product may be obtained in reduced yield due to side reactions (such as oligomerization, hydrolysis, ester exchange reactions, etc.) of the α-hydroxycarboxylic acid esters undergoing heating during reaction, distillation and the like. Moreover, since those components having lower boiling points than the α-hydroxycarboxylic acid esters are removed from the system by distillation, it is possible that reaction equilibrium is lost and side reactions are enhanced. Furthermore, the prior art methods are inadequate for integrating the synthesis and purification of α-hydroxycarboxylic acid esters to efficiently produce the desired products. Therefore, demand exists for a more efficient production method for α-hydroxycarboxylic acid esters.
Examples of α-hydroxycarboxylic acid condensates (2-hydroxycarboxylic acid condensates) include polylactic acid, polyglycolic acid, and the like. These compounds are usually obtained by subjecting α-hydroxycarboxylic acids (2-hydroxycarboxylic acids) such as lactic acid, glycolic acid, and the like, to dehydrative condensation (for example, Japanese Unexamined Patent Publication No. 1995-102044, claims). In this case, one terminal of the condensate is a hydroxyl group and the other is a carboxyl group.
Production of α-hydroxycarboxylic acid condensates by dehydrative condensation of α-hydroxycarboxylic acids poses a problem in that it is difficult to produce α-hydroxycarboxylic acid condensates of sufficient molecular weight. Therefore, in practice, such α-hydroxycarboxylic acid condensates are produced by initially preparing condensates having a molecular weight of several thousands to tens of thousands, thermally decomposing them to produce cyclic esters such as lactides and glycolides, and ring-opening polymerizing these cyclic esters.
Also known is a method in which α-hydroxycarboxylic acid condensates are produced by a dealcoholizing condensation reaction using hydroxycarboxylic acid ester such as methyl lactate, butyl lactate, and the like as starting materials. The condensates produced by this method have a hydroxyl group at one terminal and an ester group such as a methyl ester, butyl ester, or like group at the other terminal (for example, Japanese Unexamined Patent Publication 1995-173264, claims). As with the condensates derived from α-hydroxycarboxylic acids, it is difficult to produce such condensates with sufficient molecular weight, and the production process thereof is complex.
Therefore, demand exists for the development of α-hydroxycarboxylic acid condensates that have higher molecular weight than prior art condensates and a process for readily producing them.